The present invention relates to a charging apparatus and a charging method, which use electrically conductive particles to charge an object, such as an image bearing member. It also relates to a process cartridge and an image forming apparatus, which are compatible with such a charging apparatus and a charging method.
Prior to the present invention, a corona type charger (corona discharging device) has been widely used as a charging apparatus for charging (inclusive of discharging) an image bearing member (object to be charged) such as an electrophotographic photosensitive member or an electrostatic dielectric recording member to a predetermined polarity and a predetermined potential level in an image forming apparatus, for example, an electrophotographic apparatus (copying machine, printer, or the like) or an electrostatic recording apparatus.
The corona-type charging device is a noncontact-type charging device, and comprises a corona discharging electrode, such as a wire electrode, and a shield electrode which surrounds the corona discharging electrode. It is disposed so that the corona discharging opening thereof faces an image bearing member, that is, an object to be charged. In usage, the surface of an image bearing member is charged to a predetermined potential level by being exposed to discharge current (corona shower) generated as high voltage which is applied between the corona. discharging electrode and the shield electrode.
In recent years, it has been proposed to employ a contact-type charging apparatus as a charging apparatus for charging the image bearing member, that is, the object to be charged, in an image forming apparatus of low to medium speed. This is due to the fact that a contact-type charging apparatus has an advantage over a corona-type charging apparatus in terms of low ozone production, low power consumption, or the like. Also, such a contact-type charging apparatus has been put to practical use.
In order to charge an object, such as an image bearing member, with the use of a contact-type charging apparatus, the electrically conductive charging member contact-type charging member, contact-type charging device, or the like) of a contact-type apparatus is placed in contact with the object to be charged, and an electrical bias (charge bias) of a predetermined level is applied to this contact-type charging member so that the surface of the object to be charged is charged to a predetermined polarity and a predetermined potential level. The charging member is available in various forms, for example, a roller-type member (charge roller), a fur-brush-type member, a magnetic-brush type member, a blade-type member, and the like.
In reality, when an object is electrically charged by a contact-type charging member, two types of charging mechanisms (charging mechanism or charging principle: (1) mechanism which discharges electrical charge, and (2) mechanism for injecting charge) come into action. Thus, the characteristics of each contact-type charging apparatuses or methods are determined by the charging mechanism which is the dominant one of the two in charging the object.
(1) Electrical Discharge Based Charging Mechanism
This charging mechanism is a charging mechanism in which the surface of an object to be charged is charged by electrical discharge, which occurs across a microscopic gap between a contact-type charging member and the object to be charged.
In the case of the electrical-discharge-based charging mechanism, there is a threshold voltage which must be surpassed by the charge bias applied to a contact-type charging member before electrical discharge occurs between a contact-type charging member and an object to be charged, and therefore, in order for an object to be charged through the electrical discharge based charging mechanism, it is necessary to apply to the contact-type charging member a voltage with a value greater than the value of the potential level to which the object is to be charged. Thus, in principle, when the electrical discharge based charging mechanism is in action, it is impossible to avoid generating by-products of electrical discharge, that is, active ions, such as ozone ions. In reality, even a contact-type charging apparatus charges an object partially through the electrical-charge discharging mechanism as described above, and therefore, a contact-type charging apparatus cannot completely eliminate the problems caused by the active ions such as ionized ozone.
(2) Direct-charge Injection Mechanism
This is a mechanism in which the surface of an object to be charged is charged as electrical charge is directly injected into the object to be charged, with the use of a contact-type charging member. Thus, this mechanism is called xe2x80x9cdirect charging mechanismxe2x80x9d, or xe2x80x9ccharge injection mechanismxe2x80x9d. More specifically, a contact-type charging member with medium electrical resistance is placed in contact with the surface of an object to be charged to directly inject electrical charge into the surface portion of an object to be charged, without relying on electrical. discharge, in other words, without using electrical discharge in principle. Therefore, even if the value of the voltage applied to contact-type charging member is below the discharge starting voltage value, the object to be charged can be charged to a voltage level which is substantially the same as the level of the voltage applied to the contact-type charging member.
This direct injection charging mechanism does not suffer from the problems caused by the by-product of electrical discharge since it is not accompanied by ozone production. However, in the case of this charging mechanism, the state of the contact between a contact-type charging member and an object to be charged greatly affects the manner in which the object is charged, since this charging mechanism is such a mechanism that directly charges an object. Thus, this direct injection charging mechanism should comprise a contact-type charging member composed of high density material, and also should be given a structure which provides a large speed difference between the charging member and the object to be charged, so that a given point on the surface of the object to be charged makes contact with a larger area of the charging member.
A) Charging Apparatus with Charge Roller
In the case of a contact-type charging apparatus, a roller-charge system, that is, a charging system which employs an electrically conductive roller (charge roller) as a contact-type charging member, is widely used because of its desirability in terms of safety.
As for the charging mechanism in this roller charge system, the aforementioned (1) charging mechanism, which discharges electrical charge, is dominant.
Charge rollers are formed of rubber or foamed material with substantial electrical conductivity, or electrical resistance of a medium level. In some charge rollers, the rubber or foamed material is layered to obtain a specific characteristic.
In order to maintain stable contact between a charge roller and an object to be charged (hereinafter, xe2x80x9cphotosensitive memberxe2x80x9d), a charge roller is given elasticity, which in turn increases the frictional resistance between the charge roller and the photosensitive member. Also in many cases, a charge roller is rotated by the rotation of a photosensitive drum, or is individually driven at a speed slightly different from that of the photosensitive drum. As a result, problems occur: absolute charging performance declines, the state of the contact between the charge roller and the photosensitive drum becomes less desirable, and foreign matter adheres to the charge roller and/or the photosensitive member. Prior to the present invention, the dominant charging mechanism through which a roller charging member charged an object was a charging mechanism which discharged electrical charge, and therefore, even with the use of a contact-type charging apparatus, it was impossible to completely prevent the nonuniform charging of the photosensitive member.
FIG. 5 is a graph which shows an example of efficiency in contact-type charging. In the graph, the abscissas represents the bias applied to a contact-type charging member, and the ordinate represents the potential levels corresponding to the voltage values of the bias applied to the contact-type charging member. The characteristics of the charging by a roller are represented by a line designated by a character A. According to this line, when a charge roller is used to charge an object, the charging of an object occurs in a voltage range above an electric discharge threshold value of approximately xe2x88x92500 V. Therefore, generally, in order to charge an object to a potential level of xe2x88x92500 V with the use of a charge roller, either a DC voltage of xe2x88x921,000 V is applied to the charge roller, or an AC voltage with a peak-to-peak voltage of 1,200 V, in addition to a DC voltage of xe2x88x92500 V, is applied to the charge roller to keep the difference in potential level between the charge roller and the object to be charged, at a value greater than the electric-discharge threshold value, so that the potential of the photosensitive drum converges to the desired potential level.
More specifically, in order to charge a photosensitive drum with a 25 xcexcm thick organic photoconductor layer by pressing a charge roller upon the photosensitive member, charge bias with a voltage value of approximately 640 V or higher should be applied to the charge roller. Where the value of the charge bias is approximately 640 V or higher, the potential level at the surface of the photosensitive member is proportional to the level of the voltage applied to the charge roller; the relationship between the potential level and the voltage applied to the charge roller is linear. This threshold voltage is defined as a charge start voltage Vth.
In other words, in order to charge the surface of a photosensitive member to a potential level of Vd which is necessary for electrophotography, a DC voltage of (Vd+Vth), which is higher than the voltage level to which the photosensitive member is to be charged, is necessary. Hereinafter, the above described charging method in which only DC voltage is applied to a contact-type charging member to charge an object will be called a xe2x80x9cDC charging methodxe2x80x9d.
However, prior to the present invention, even with the use of the DC charging method, it was difficult to bring the potential level of a photosensitive member exactly to a target level, since the resistance value of a contact-charging member changed due to changes in ambience or the like, and also the threshold voltage Vth changed as the photosensitive member was shaved away.
As for a counter measure for the above described problem, Japanese Laid-Open Patent Application No. 149,669/1988 discloses an invention which deals with the above problem to effect more uniform charging of a photosensitive member. According to this invention, an xe2x80x9cAC charging methodxe2x80x9d is employed, in which a compound voltage composed of a DC component equivalent to a desired potential level Vd, and an AC component with a peak-to-peak voltage which is twice the threshold voltage Vth, is applied to a contact-type charging member. This invention is intended to utilize the averaging effect of alternating current. According to this invention, the potential of an object to be charged is caused to converge to the Vd, that is, the center of the peaks of the AC voltage, without being affected by external factors, such as operational ambience.
However, even in the case of the contact-type charging apparatus in the above described invention, the principal charging mechanism is a charging mechanism which uses electrical discharge from a contact-type charging member to a photosensitive. member. Therefore, as already described, the voltage applied to the contact-type charging member needs to have a voltage level higher than the voltage level to which the photosensitive member is to be charged. Thus, ozone is generated, although only in a small amount.
Further, when AC current is used so that an object is uniformly charged due to the averaging effect of AC current, the problems related to AC voltage become more conspicuous. For example, more ozone is generated; noises traceable to the vibration of the contact-type charging member and the photosensitive drum caused by the electric field of AC voltage increase; and the deterioration of the photosensitive member surface caused by electrical discharge increases, which add to the prior problems.
B) Charging Apparatus with Fur Brush
In the case of this charging apparatus, a charging member (fur-brush-type charging device) with a brush portion composed of electrically conductive fiber is employed as the contact-type charging member. The brush portion composed of electrically conductive fiber is placed in contact with a photosensitive member as an object to be charged, and a predetermined charge bias is applied to the charging member to charge the peripheral surface of the photosensitive member to a predetermined polarity and a predetermined potential level.
Also in the case of this charging apparatus with a fur brush, the dominant charging mechanism is the electrical-discharge based charging mechanism.
It is known that there are two type of fur-brush-type charging devices: a fixed type and a roller type. In the case of the fixed type, fiber with medium electrical resistance is woven into foundation cloth to form pile, and a piece of this pile is adhered to an electrode. In the case of the rotatable type, the pile is wrapped around a metallic core. In terms of fiber density, pile with a density of 100 fiber/cm2 can be relatively easily obtained, but the density of 100 fiber/cm2 is not sufficient to create a state of contact which is satisfactory to charge an object by charge injection. Further, in order to give a photosensitive member satisfactorily uniform charge by charge injection, a velocity difference which is almost impossible to attain with the use of a mechanical structure must be established between a photosensitive drum and a roller type fur brush. Therefore, the fur-brush-type charging device is not practical.
The relationship between the DC voltage applied to a fur-brush-type charging member and the potential level to which a photosensitive member is charged by the DC voltage applied to the fur brush shows a characteristic represented by a line B in FIG. 5. As is evident from the graph, also in the case of the contact-type charging apparatus which comprises a fur brush, whether the fur brush is of the fixed type or the roller type, the photosensitive member is charged mainly through electrical discharge triggered by applying to the fur brush a charge bias the voltage level of which is higher than the potential level desired for the photosensitive member.
C) Magnetic-Brush-type Charging Apparatus
A charging apparatus of this type comprises a magnetic brush portion (magnetic brush based charging device) as the contact-type charging member. A magnetic brush is constituted of electrically conductive magnetic particles magnetically confined in the form of a brush by a magnetic roller or the like. This magnetic brush portion is placed in contact with a photosensitive member as an object to be charged, and a predetermined charge bias is applied to the magnetic brush to charge the peripheral surface of the photosensitive member to a predetermined polarity and a predetermined potential level.
In the case of this magnetic brush type charging apparatus, the dominant charging mechanism is the charge injection mechanism (2).
As for the material for the magnetic brush portion, electrically conductive magnetic particles, the diameters of which are in a range of 5-50 xcexcm, are used. With the provision of a sufficient difference in peripheral velocity between a photosensitive drum and a magnetic brush, the photosensitive member can be uniformly charged through charge injection.
In the case of a magnetic brush type charging apparatus, the photosensitive member is charged to a potential level which is substantially equal to the voltage level of the bias applied to the contact-type charging member, as shown by a line C in FIG. 5.
However, a magnetic brush type charging apparatus also has its own problems. For example, it is complicated in structure. Also, the electrically conductive magnetic particles, which constitute the magnetic brush portion, become separated from the magnetic brush and adhere to a photosensitive member.
Japanese Patent Publication Application No. 3,921/1994 discloses a contact-type charging method, according to which a photosensitive member is charged by injecting electric charge into the charge injectable surface layer thereof, more specifically, into the traps or electrically conductive particles in the charge injectable surface layer. Since this method does not rely on electrical discharge, the voltage level necessary to charge the photosensitive member to a predetermined potential level is substantially the same as the potential level to which the photosensitive member is to be charged, and in addition, no ozone is generated. Further, since AC voltage is not applied, there is no noise traceable to the application of AC voltage. In other words, a magnetic-brush type charging system is an excellent charging system superior to the roller-type charging system in terms of ozone generation and power consumption, since it does not generate ozone, and uses far less power compared to the roller-type charging system.
D) Toner Recycling Process (Cleanerless System)
In a transfer type image forming apparatus, the toner which remains on the peripheral surface of a photosensitive member (image bearing member) after image transfer is removed by a cleaner (cleaning apparatus) and becomes waste toner. Not only for obvious reasons, but also for environmental protection, it is desirable that the waste toner is not produced. Thus, image forming apparatuses capable of recycling toner have been developed. In such an image forming apparatus, a cleaner is eliminated, and the toner which remains on the photosensitive member after image transfer is removed from the photosensitive drum by a developing apparatus; the residual toner on the photosensitive member is recovered by a developing apparatus at the same time as a latent image on the photosensitive drum is developed by the developing apparatus, and then is reused for development.
More specifically, the toner which remains on a photosensitive member after image transfer is recovered by fog removal bias (voltage level difference Vback between the level of the DC voltage applied to a developing apparatus and the level of the surface potential of a photosensitive member) during the following image transfer. According to this cleaning method, the residual toner is recovered by the developing apparatus and is used for the following image development and thereafter; the waste toner is eliminated. Therefore, the labour spent for maintenance is reduced. Further, being cleanerless is quite advantageous in terms of space, allowing image forming apparatuses to be substantially reduced in size.
E) Coating of Contact-type Charging Member with Electrically Conductive Powder
Japanese Laid-Open Patent Application No. 103,878/1991 discloses a contact-type charging apparatus with such a structure that coats a contact-type charging member with electrically conductive powder, on the surface which comes in contact with the surface of an object to be charged, so that the surface of the object to be charged is uniformly charged, that is, without irregularity in charge. The contact-type charging member in this charging apparatus is rotated by the rotation of the object to be charged, and the amount of ozone generated by this charging apparatus is remarkably small compared to the amount of ozonic products generated by a corona type charging apparatus such as SUKOROTRON. However, even in the case of this charging apparatus, the principle, based on which an object is charged, is the same as the principle, based on which an object is charged by the aforementioned charge roller; in other words, an object is charged by electrical discharge. Further, also in the case of this charging apparatus, in order to assure that an object to be charged is uniformly charged, compound voltage composed of DC component and AC component is applied to the contact-type charging member, and therefore, the amount of ozonic products traceable to electrical discharge becomes relatively large. Thus, even this contact-type charging apparatus is liable to cause problems; for example, images are affected by ozonic products, appearing as if flowing, when this charging apparatus is used for an extended period of time, in particular, when this charging apparatus is used in a cleanerless image forming apparatus for an extended period of time.
As described in the preceding paragraphs regarding the technologies prior to the present invention, it is difficult to directly charge an object with the use of a contact-type charging apparatus with a simple structure which comprises a contact-type charging member, such as a charge roller or a fur brush. Also in the case of an image forming apparatus which employs such a charging apparatus, the photosensitive member is liable to be insufficiently charged, causing images to appear foggy (during reversal development, toner is adhered to the areas which are supposed to remain white), or the photosensitive member is liable to be nonuniformly charged, causing images to be appear irregular in terms of continuity.
In the case of the contact-type charging apparatus structured so that the contact-type charging member is coated with electrically conductive powder, on the surface which comes in contact with the surface of the object to be charged, so that the contact-type charging member is rotated by the rotation of the photosensitive member, and so that the photosensitive member is mainly charged by electrical discharges, ozonic products are liable to be accumulated, and; images are affected by the accumulated ozonic products, appearing as if flowing, when such a charging apparatus is used for an extended period of time, in particular, when such a charging apparatus is used in a cleanerless image forming apparatus for an extended period of time.
Further, in the case of the cleanerless image forming apparatus, there is the problem that the residual toner causes the photosensitive member to be unsatisfactorily charged in a charging portion.
Further, U.S. Pat. No. 5,432,037 discloses an invention in which electrically conductive particles are mixed into developer so that even if developer adheres to a charger roller, the charging operation is not interfered with. However, also in this case, a photosensitive member is primarily charged through electrical discharge, and therefore, there are problems similar to those described above.
Accordingly, a primary object of the present invention is to provide a charging apparatus and a charging method, which are capable of uniformly charging an object, with the use of only a simple charging member such as a charge roller, a fiber brush, or the like, and also remain reliable for a long period of time.
Another object of the present invention is to provide a charging apparatus and a charging method, in which the voltage applied to a charging member is reduced so that an object can be charged without generating ozone.
Another object of the present invention is to provide a charging apparatus and a charging method, in which charge is injected into an object from an inexpensive charging member.
Another object of the present invention is to provide a charging apparatus and a charging method, which do not create problems traceable to ozonic products.
Another object of the present invention is to provide a contact type charging apparatus and a charging method, which do not generate the charging noises.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings.